The infrared spectrum covers a range of wavelengths longer than the visible wavelengths but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. Infrared wavelengths extend from 0.75 micrometers to 1 millimeter. The function of an infrared detector is to respond to energy of a wavelength within some particular portion of the infrared region.
Heated objects will dissipate thermal energy having characteristic wavelengths within the infrared spectrum Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. No single detector is uniformly efficient over the entire infrared frequency spectrum. Thus, detectors are selected in accordance with their sensitivity in the range corresponding to the particular detection function of interest to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detector must also be selected in view of the intended detection function.
A variety of different types of infrared detectors have been proposed in the art since the first crude infrared detector was constructed in the early 1800's. Virtually all contemporary infrared detectors are solid state devices constructed of materials that respond to infrared energy in one of several ways. Thermal detectors respond to infrared energy by absorbing that energy and thus causing an increase in temperature of the detecting material. The increased temperature in turn causes some other property of the material, such as resistively, to change. By measuring this change the infrared radiation can be derived.
Photo-type detectors (e.g., photoconductive and photovoltaic detectors), absorb the infrared frequency energy directly into the electronic structure of the material, inducing an electronic transition which leads to a change in the electrical conductivity (photoconductors) or to the generation of an output voltage across the terminals of the detector (photovoltaic detectors). The precise change that is affected is a function of various factors including the particular detector material selected, the doping density of that material and the detector area.
By the late 1800's, infrared detectors had been developed that could detect the heat from an animal at one quarter of a mile. The introduction of focusing lenses constructed of materials transparent to infrared frequency energy, advances in semiconductor materials and the development of highly sensitive electronic circuitry have advanced the performance of contemporary infrared detectors close to the ideal photon limit.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements, the outputs of which are connected to sophisticated processing circuitry. By rapidly analyzing the pattern and sequence of detector element excitation, the processing circuitry can identify and monitor sources of infrared radiation.
Though the theoretical performance of such systems is satisfactory for many applications, it is difficult to actually construct structures that mate a million or more detector elements and associated circuitry in a reliable and practical manner. Consequently, practical applications for contemporary infrared detection systems have necessitated that further advances be made in areas such as miniaturization of the detector array and accompanying circuitry, minimization of noise intermixed with the electrical signal generated by the detector elements, and improvements in the reliability and economical production of the detector array and accompanying circuitry.
A contemporary subarray of detectors may, for example, contain 256 detectors on a side, or a total of 65,536 detectors, the size of each square detector being approximately 0.0035 inches on the side with 0.0005 inches spacing between detectors. Such a subarray would therefore be 1.024 inches on a side. Thus, interconnection of such a subarray to processing circuitry requires a connective module with sufficient circuitry to connect each of the 65,536 detectors to processing circuitry within a square, a little more than one inch on a side. The subarray may, in turn, be joined to form an on-focal plane array that connects to 25 million detectors or more. Considerable difficulties are presented in aligning the detector elements with conductors on the connecting module and in isolating adjacent conductors in such a dense environment.
The outputs of the detectors must undergo a series of processing steps in order to permit derivation of the desired information. The more fundamental processing steps include preamplification, tuned bandpass filtering, clutter and background rejection, multiplexing and fixed noise pattern suppression. By providing a signal processing module that performs at least a portion of the processing functions within the module, i.e. on integrated circuit chips disposed adjacent the detector focal plane, the signal from each detector need be transmitted only a short distance before processing. As a consequence of such on focal plane or up front signal processing, reductions in size, power and cost of the main processor may be achieved. Moreover, up front signal processing helps alleviate performance, reliability and economic problems associated with the construction of millions of closely spaced conductors connecting each detector element to the main signal processing network.
Infrared detectors are typically fabricated from monolithic semiconductor wafers by photolithographic techniques. The processed wafers are diced to form smaller arrays having various pixel sizes, such as 32.times.32 or 8.times.128. A plurality of diced arrays are attached to the signal processing modules. Many signal processing modules can be assembled to form a focal plane array.
Backside-illuminated detectors are well known. Backside-illuminated detectors have an electrical contact fabricated upon the front side, thus necessitating illumination through the substrate. The focal plane array may be formed by attaching the diced arrays to signal processing modules by flip-chip bump bonding.
In flip-chip bump bonding, the electrical contacts formed upon the front surface of the array are comprised of a soft, malleable conductive material such as indium. The indium bump bonds are electrically connected to corresponding contacts formed upon the signal processing module by flipping or inverting the infrared detector array such that the indium bump bonds contact the corresponding electrical contacts of the signal processing module. An epoxy filler may be used to secure the infrared detector arrays upon the signal processing module.
Because of the brittle nature of the infrared detector array, a thin substrate material, such as cadmium telluride (CdTe) or sapphire is typically attached to the back side of the infrared detector array prior to flip-chip bump bonding. The substrate provides mechanical support to the array and thereby facilitates handling.
A particular problem with the attachment of detector arrays to surfaces such as the contacts of the signal processing module is alignment. The back side or illuminated surface of the CdTe or sapphire substrate is polished to provide a smooth surface. The smooth surface is required to provide maximum absorption and transmission of incident infrared radiation to the detector elements. Therefore, no features exist upon the back surface of the substrate to indicate the exact positions of the detector element pixels of the infrared detector array beneath.
In the prior art, the detector pixel locations are determined by measuring the distance between the pixels and the array edges of the front side. However, once an array is positioned for attachment to a signal processing module, only the back side is visible. Therefore, the edges of the back side must be used as an indirect reference. That is, since the front side edges of the infrared detector array to which the initial measurement was made are not visible, the back side edges must be used instead. The rear edges of the diced array may not be true to the front edges, e.g. due to tapers or chips in the substrate, therefore the accuracy of using the back edges as references is quite limited.
The most accurate dicing equipment today has an average tolerance of .+-.5 microns. In combining the errors due to imperfections on the edges with the accuracy of dicing, an overall accuracy of using the back side edges to locate the pixel is estimated to be approximately .+-.10 microns.
It is also necessary to know the precise locations of the pixel elements of infrared detector arrays after they have been attached to signal processing modules. This is required when the signal processing modules are being assembled into a staring focal plane array. The pixels must be aligned when the signal processing modules are formed into a focal plane array. Precise placement of the detector pixels is necessary to achieve the intended performance.
Placement of the pixels must be identifiable from the back side of the arrays both before and after they are attached to the signal processing electronics in order to obtain the required performance. Therefore, it would be desirable to provide a means for obtaining the precise locations of the pixel elements formed upon the front side of the detector array by visually observing the back side of the array.